Electrical bushing having a stress relieving shield and method of constructing same

ABSTRACT

An electrical bushing having a corona shield and methods of constructing such a bushing. The bushing contains an opening through an epoxy insulator. The opening is lined with a shield having a thin conducting layer, such as a layer of aluminum, disposed on a thin insulating layer, such as a film of highly polymeric polyethylene terephthalate. The shield is wrapped around a mold plug and then epoxy is disposed around the plug. After the epoxy has cured, the mold plug is withdrawn to leave the shield attached to the wall of the molded opening. In another method of constructing a shield, a coating of conductive material is sprayed onto a mold plug which is placed into a mold. The mold plug is removed after the epoxy injected into the mold has cured and the conductive material is left as a coating on the inside of the opening provided by the mold plug.

United States Patent 1191 Martincic et al.

[ 1 Jan. 1,1974

ELECTRICAL BUSHING HAVING A STRESS [54] 1,105,018 4/1961 Germany 174/152 R RELIEVING SHIELD AND METHOD 0F 1 g t g it i rea r1 am CONSTRUCTING SAME 320,903 5/1957 Switzerland 174/142 [75] Inventors: Paul W. Martincic, Sharpsville;

James F. Quirk, Penn Hills, both of Primary Examiner-Laramie E Askin Attorney-A. T.' Stratton et al. [73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

221 Filed: Oct. 30, 1972 ABSTRACT PP 302,329 An electrical bushing having a corona shield and -methods of constructing such a bushing. The bushing [52] us. c1 174/142, 29/631, 264/104 an Opening through ePXY insulamr- The 264/255 339/278 A, 2644104105 .2 opening is lined with a shield having a thin conducting [51 Int. Cl 1101b 17/26, HOlb 15/00 layer Such as a layer 0f aluminum (filmed a [58] Field of Search 174/18, 31 R, 73 R, insulating layer, Such as a film P [74/73 SC 140 R 140 C 142, 143, 152 R polyethylene terephthalate. The shield is wrapped 153 R; 29/ 3 I. 339/275 c 27 C 277 C, around a mold plug and then epoxy 18 disposed around 278 C the plug. After the epoxy has cured, the mold plug is withdrawn to leave the shield attached to the wall of [56] References Cited the molded opening. ln another method of constructing a shield, a coating of conductive material is UNITED STATES PATENTS sprayed onto a mold plug which is placed into a mold. 2,870,524 1/1959 Klnncar 264/255 X The mold plug is removed after the epoxy injected gz 'x zrg gi 22 32 into the mold has cured and the conductive material is 3:320:341 5/1967- Mackie .I.IIII,,....IIIIIII't4/255 x t fii 133115; the inside the Pening Provided FOREIGN PATENTS OR APPLICATIONS 65,960 3/1969 Germany 174/142 7 Claims, 9 Drawing Figures IE. m mn SHIELD POLYETHYLENE TEREPHTHALATE PATENTEU JAN 1 I974 SHEET 1 BF 2 FIG. 2B

FIGZA POLYETHYLENE /TEREPHTHALATE BZ-ALUMINUM PATENTEDJAH 1 I974 3. 783. 1 81 SHEET 2 OF 2 WRAP sPRAY %Y5 igfihg CONDUCTIVE 1 FROM SHIELD MOLD PLUG 38g g S g V Y Y SOLDER INJECT INJECT WIRE TO EPOXY IN EPOXY IN FASTENER MOLD AROUND MOLD AROUND MOI-D PLUG MOLD PLUG V Flig'g R CURE R TOSHIELD EPOXY EPOXY REMOVE PLUG REMOvE PLUG AND MOLD AND MOLD FROM FROM CURED EPOXY CURED EPOXY FIG? FIG.8

ELECTRICAL BUSHING HAVING A STRESS RELIEVING SHIELD AND METHOD OF CONSTRUCTING SAME BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates, in general, to electrical bushing assemblies and, more specifically, to epoxy bushing insulators having stress relieving shields and methods of constructing such insulators.

2. Description of the Prior Art Electrical bushings of the draw-lead type include an insulator member with a longitudinal opening extending therethrough. The purpose of the opening is to permit the placement of a conductor lead or stud within the insulator to conduct current through the bushing. In paper wound bushings, the opening is provided by a hollow metal tube which also provides a mandrel on which the paper is wound. In cast epoxy bushings, the opening is provided by a suitable hollow metal tube which is encapsulated by the material comprising the bushing insulator.

The metal tubes are usually constructed of copper or aluminum and have sufficient rigidity to support the paper wound thereon in paper wound bushings or to maintain an opening during the casting process in epoxy bushings. In epoxy bushings, small corona generating voids may exist between the metal tube and the epoxy material due to gas entrapment or irregularities on the surface of the metal tube. To eliminate corona problems, a corona shield, or electrical stress relieving shield, is cast into the epoxy around the metal conductor tube to eliminate the electrical discharges between the metal conductor tube and the epoxy. lt is desirable, and it is an object of this invention, to eliminate the need for the metal support tube in epoxy bushings. It is also desirable, and it is a further object of this invention, to eliminate the need for a separate shield embedded in the epoxy and to provide a bushing which is substantially corona free and unaffected by thermal expansion characteristics.

SUMMARY OF THE INVENTION There is disclosed herein new and useful epoxy bushing apparatus and construction methods which conveniently provide a corona shield in the bushing. In one embodiment of this invention, a shield is wrapped around a mold plug and epoxy material is cast-therearound. The mold plug is withdrawn from the epoxy after the epoxy has cured, thus leaving an opening in the cast epoxy bushing insulator which is lined with the shield. The shield includes a layer of film, a layer of conductive material which is vacuum deposited on the film, and a layer of adhesive on the conductive material. The adhesive side of the shield is positioned adjacent to the epoxy. The shield is connected to a conductor stud by a shield lead soldered to a fastener which is attached to the conductive material of the shield. In another embodiment of this invention, a conductive material is sprayed onto a mold plug which is cast into the epoxy insulator. When the epoxy has cured, the mold plug is removed, thereby leaving a conductive material on the wall of the opening in the epoxy insulator formed by the mold plug.

A metal support tube is not required in bushings which are constructed as taught by this invention. The opening is formed by the mold plug. The electrical stress relief is provided by the conductive material. Due to the excellent adhesion of the epoxy to the shield, corona discharges are substantially reduced. Since the shield is relatively thin, the thermal expansion characteristics of the bushing are improved.

BRIEF DESCRIPTION OF THE DRAWING Further advantages and uses of this invention will become more apparent when considered in view of the following detailed description and drawing, in which:

FIG. 1 is an elevational view, partly in section, of an electrical bushing having a corona shield attached to an opening therethrough;

FIG. 2A is a view of the bushing insulator and molding member;

FIG. 2B is an enlarged view of a corona shield;

FIG. 3 is a view of a mold plug with a corona shield partially wrapped therearound;

FIG. 4 is a view of a mold plug with a coating of conductive material partially sprayed thereon;

FIG. 5 is a view of a corona shield with a portion thereof enlarged for clarity; and

FIGS. 6, '7 and 8 are block diagrams illustrating steps in the construction of the electrical bushings of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the following description, similar reference characters refer to similar members in all Figures of the drawing.

Referring now to the drawing, and FIG 1 in particular, there is shown an electrical bushing assembly constructed according to this invention. An insulator 10 is constructed of an epoxy material and includes the longitudinal opening 12 and the mounting flange 14. A conductor stud 16 is located within the opening 12 and is secured to the end of the insulator by nuts 18 and 20 with the aid of the caps 22 and 24. It is within the contemplation of this invention that the conductor stud 16 may be replaced by a suitably flexible conductor lead since the main function of the conductor stud 16 is to carry current.

A shield 26, which reduces corona producing electrical stresses, is located adjacent to the wall of the opening 12. Since the shield 26 is electrically connected to the stud 16 by the shield lead 28, negligible electrical stresses exist between the surface of the stud 16 and the shield 26, thus corona or partial discharges are not a problem in this region. The shield 26 is extremely smooth and contains substantially no irregularities, thus the epoxy material of the insulator adheres uniformly to the shield 26 and corona producing voids are virtually nonexistent.

, FIG. 2A illustrates a mold arrangement which may be used in constructing the bushing shown in FIG. 1. As shown in FIG. 2B, the shield includes a layer of highly polymeric polyethylene terephthalate film 30 which is sold under the trademark of Mylar, a layer of electrically conductive material 32, such as aluminum, and a suitable epoxy adhesive 34, such as a bisphenol A type. The shield 26 is constructed by vacuum depositing a thin layer of the conductive material 32 on the substrate film 30 and coating the conductive material 32 with the adhesive 34. The shield is approximately 0.0015 mils thick and does not contain any significant irregularities. The film 30 protects the conducting material 32 from being scuffed when the stud 16, or a lead, is drawn through the opening 12. The adhesive 34 tenaciously attaches the epoxy insulator to the smooth conducting material 32 without producing any voids therebetween as the epoxy adhesive 34 cures during 4 the curing of the epoxy insulator 10.

The shield 26 is positioned around a mold plug 36. The mold plug illustrated in FIG. 2A includes a metal shaft 38 with a tube 4t) disosed thereon for allowing easy removal of the mold plug 36. The tube 40 may be constructed of polytetrafluoroethylene which is sold under the trademark of Teflon. Other arrangements for the mold plug 36 may be used, such as a solid Teflon mold plug. A mold 42 is positioned around the mold plug 36 and is formed 'to properly shape the outside surface of the insulator l0.

During the casting process, the epoxy material comprising the insulator 10 is injected into the region located between the mold 42 and the mold plug 36. The heat and pressures existing during the casting operation are sufficient to provide a voidless bond between the epoxy and the adhesive 34. When the epoxy material has cured, the mold plug 36 is withdrawn from the epoxy leaving the conductive material 32 in the opening 12 to provide a corona shield 26.

The operation of wrapping the shield 26 around the mold plug 36 is shown in FIG. 3. The shield 26 is folded or wrapped around the mold plug 36 with a sufficient overlapping of the shield 26 to prevent any gaps therein. In FIG. 3, the upper portion of the shield 26 is shown in a partially unwrapped position to illustrate the overlapping of the shield 26. Other wrapping arrangements, such as spiral wrapping, may be used within the contemplation of this invention. FIG. 7 is a block diagram which illustrates the major steps involved in constructing a bushing according to this embodiment.

Other arrangements may be used to dispose a conductive material around the mold plug 36. FIG. 4 shows a conductive material 33 being sprayed onto the mold plug 36. The coating of conductive material 33 can be applied by flame or plasma spraying of a suitable material, such as tin or aluminum, or by spraying a conductive ink material onto the mold plug 36. The rough external surface of the coating of conductive material 33 aids adhesion of the conductive material 33 to the epoxy when the mold plug is withdrawn. FIG. 8 is a block diagram illustrating the major steps performed in constructing a bushing according to this embodiment.

FIG. 5 shows a portion of the shield 26 with the shield lead 28 attached thereto. The lead 28 is soldered to a snap fastener 46 which is attached to the conductive material 32. The enlarged portion of FIG. 5 illustrates the different layrs comprising the shield 26.

The process of attaching the lead 28 to the conductive material 32 involves several steps. The adhesive 34 is removed from the conductive material 32 by a proper solvent, such as alcohol. The lead 28 is soldered to one portion of the fastener 46. A hole is punched in the conductive material 32 and in the film 30. A portion of the fastener 46 is inserted through the hole and engaged with another part of the fastener 46 located on the other side of the shield. FIG. 6 is a block diagram illustrating the major steps performed in connecting the lead 28 to the shield 26.

The embodiments of this invention provide several advantages over prior art bushings and bushing construction methods. Since the only purpose of a metal support tube was to maintain an opening through the epoxy insulator, a bushing constructed according to this invention does not require the extra cost of a metal support tube. Additionally, by elimination of the support tube, the thermal expansion characteristics of tbe bushings are improved. Corona discharges herebefore existing in voids between the support tube and the epoxy are greatly reduced due to the tight adherence of the epoxy to the corona shield and the smooth surface of the layer of conductive material. Providing the corona shield is accomplished in a manner which requires a minimum number of steps.

Since numerous changes may be made in the abovedescribed apparatus and methods, and since different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all of the matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative rather than limiting.

We claim as our invention: 1. A method of constructing an epoxy bushing insulator comprising the steps of:

wrapping a conductive sheet around a longitudinal mold plug, said conductive sheet comprising an in sulating substrate material with a conductive layer disposed thereon, with said insulating substrate material facing said mold plug; injecting epoxy between the wrapped mold plug and a mold positioned around said mold plug;

curing the epoxy; and

removing said mold and mold plug from the cured epoxy thereby leaving the conductive sheet attached to the wall of an opening existing in the epoxy when the mold plug is removed.

2. The method of constructing an epoxy bushing insulator of claim 1 wherein an adhesive material is dis posed on said conductive layer.

3. The method of constructing an epoxy bushing insulator of claim 1 including the steps of:

soldering a wire lead to a snap fastener; and

attaching the snap fastener to the conductive sheet.

4. An electrical bushing comprising an insulator constructed of epoxy and having a longitudinal opening extending therethrough, a stress relieving shield which is tightly attached to the wall of said opening, said shield comprising a sheet of insulating material and a layer of conductive material each having first and second sides, the first sides of said insulating sheet and conductive layers being tightly adhered to each other, the second side of said sheet of insulating material facing the center of said opening, and a conducting means located in said opening for conducting current through said bushing, said conducting means being smaller than said opening thereby providing a substantial space between the conducting means and the sheet of insulating material which does not contain any other solid insulating material.

5.-The electrical bushing of claim 4 wherein the second side of the layer of conductive material is tightly attached to the epoxy by an adhesive.

6. The electrical bushing of claim 5 wherein the adhesive comprises a bisphenol A type epoxy adhesive material.

7. The electrical bushing of claim 4 wherein the insulating material comprises highly polymeric polyethylene terephthalate and the conductive material comprises aluminum. 

2. The method of constructing an epoxy bushing insulator of claim 1 wherein an adhesive material is disposed on said conductive layer.
 3. The method of constructing an epoxy bushing insulator of claim 1 including the steps of: soldering a wire lead to a snap fastener; and attaching the snap fastener to the conductive sheet.
 4. An electrical bushing comprising an insulator constructed of epoxy and having a longitudinal opening extending therethrough, a stress relieving shield which is tightly attached to the wall of said opening, said shield comprising a sheet of insulating material and a layer of conductive material each having first and second sides, the first sides of said insulating sheet and conductive layer being tightly adhered to each other, the second side of said sheet of insulating material facing the center of said opening, and a conducting means located in said opening for conducting current through said bushing, said conducting means being smaller than said opening thereby providing a substantial space between the conducting means and the sheet of insulating material which does not contain any other solid insulating material.
 5. The electrical bushing of claim 4 wherein the second side of the layer of conductive material is tightly attached to the epoxy by an adhesive.
 6. The electrical bushing of claim 5 wherein the adhesive comprises a bisphenol A type epoxy adhesive material.
 7. The electrical bushing of claim 4 wherein the insulating material comprises highly polymeric polyethylene terephthalate and the conductive material comprises aluminum. 